Fiber lasers are an important new class of lasers that provide significant advantages of efficiency and practicality in comparison with other laser types such as free-space lasers. With the advent of a double-clad fiber (DCF), fiber lasers have been scaled to kilowatt (kW) power levels. In a DCF, pump light propagates in a relatively large inner cladding, typically 125 to 600 micrometers in diameter, and the laser light propagates in the much smaller core, typically 5 to 100 micrometers in diameter. The core is doped with ions of a rare-earth element, such as Ytterbium, and is surrounded by the inner cladding, which guides the pump light to be absorbed in the doped core for laser light amplification along the entire fiber length. Ideally, at the output of the laser, no light will be propagating in the inner cladding, and all of the output laser beam will originate from the core. In some fiber laser systems, it is similarly desirable to have exclusively core light propagating between components or between amplification stages.
In practice, the output of a fiber laser or amplifier based on a DCF consists of some core light and some cladding light. The cladding light may contain residual unabsorbed pump light and any laser light that has escaped from the core into the cladding e.g. due to scattering or spontaneous emission in the core. The cladding light may contain optical beams at a large range of divergence angles and a variety of wavelengths, depending on their source(s) and the construction of the laser system. The cladding light is deleterious for a number of applications, and should preferably be removed, or “stripped”, from the fiber. For high-power fiber sources, more than 300 W of cladding light may be present, and safely and efficiently removing this light represents a significant technological challenge. Typically, the stripped cladding light is converted to heat, and care must be taken to avoid overheating fiber coatings or other components such as ferrules, splice protectors, and the like. Fiberoptic components frequently contain polymers with a limited operating temperature range, e.g. less than 85° C. maximum continuous operating temperature for some common fiber-optic polymers. To obtain a high light stripping efficiency, the stripped cladding light must be prevented from re-entering the inner cladding. Furthermore, the device used to strip the cladding light should not introduce optical losses or otherwise perturb light propagating in the fiber core.
Most prior-art cladding light strippers (CLS) use a thin layer of a high index polymer, which is applied to the cladding to “un-guide” cladding light. For example, Vilhelmsson in U.S. Pat. No. 4,678,273; Pratt in U.S. Pat. No. 6,865,316; and Frith in U.S. Pat. Nos. 8,027,557 and 8,229,260 disclose devices for stripping cladding light, which operate by coupling to the cladding a layer or layers that are index-matched to the cladding, or have a refractive index higher than the refractive index of the cladding.
Referring to FIG. 1A, an index-matching cladding light stripper 10A includes an optical fiber 11 having a core 19, a cladding 12, and a coating 13, which is stripped from the cladding 12 in a middle area 14 of the optical fiber 11. A high-index polymer layer 15 is applied to the cladding 12 in the middle area 14. In operation, cladding light 16 is guided by the cladding 12. When the cladding light 16 is coupled to the high-index polymer layer 15 in the middle area 14, the cladding light 16 is coupled out of the cladding 12, as shown in FIG. 1A.
The index-matching cladding light stripper 10A can sometimes achieve good efficiency of stripping cladding light, yet its optical power scalability is limited by the highest temperature the high-index polymer layer 15 can handle, typically in the range of 100° C. to 150° C. Scaling up cladding light power using high-index or index-matched layers is challenging and limited, because using high index polymer to strip out the light has no or little ability to control stripping rate. Hence, the power handling capability of the index-matching cladding light stripper 10A is limited by localized heating.
In several prior art systems, refractive index or the thickness of polymer is selected to facilitate more even temperature distribution. For example, Meleshkevich et al. in U.S. Pat. No. 7,839,901 disclose a polymer coating having a refractive index that decreases with temperature. The polymer is index-matched to a cladding it is coated upon. When the polymer overheats due to absorption of released cladding light, its refractive index decreases, thereby limiting the local release of light from the cladding and the resultant heating, causing the cladding light to be released at some location downstream of the overheated point. As a result, the heat release becomes more uniform.
Optical and thermal properties of polymer-based cladding strippers, such as absorption of IR radiation, spectral dependence, heating rate, and thermal damage threshold, contribute to limiting the maximum cladding light power that can be stripped to approximately 100 W. In a practical fiber laser system, cladding light usually includes high numerical aperture (NA) residual pump light and low NA scattered core light. The low NA light is difficult to remove with polymer based cladding light strippers, since the strip rate of these strippers is very sensitive to NA of the light. High NA light tends to strip out in a much shorter distance compared to low NA light. Heat load of polymer based cladding light strippers is highly non-uniform, and extra length must be used to achieve desired strip rate for the low NA cladding light.
Langseth et al. in US Patent Application Publication 2012/0070115 and Majid et al. in a PCT application WO 2012088267 disclose optical fibers having a roughened outer surface of the cladding, to scatter the light out of the cladding. By way of example, referring to FIG. 1B, a roughened-surface cladding light stripper 10B includes the optical fiber 11 having the core 19, the cladding 12, and the coating 13, which is stripped off the cladding 12 in the middle area 14 of the optical fiber 11. An outer surface 18 of the cladding 12 is roughened in the exposed middle area 14. In operation, the cladding light 16 is guided by the cladding 12. When the cladding light 16 is coupled to the roughened outer surface 18, the cladding light 16 is scattered out of the cladding 12 as shown in FIG. 1B.
One advantage of this approach is that the cladding stripper can be polymer free. Detrimentally, most of the cladding light is stripped in the upstream portion of the cladding stripper, creating uneven temperature distribution in the stripper. Furthermore, roughening the surface may generate micro-cracks that can propagate over time and cause the fiber to fail.